Hardest weapons to easiest weapons to use:

My personal ranking:
1. Shard Bombs: With the right placement, you can dish out almost 600-700 damage per bomb. Knockback and blast radius of crystal bombs makes this bomb very difficult to maximize its damage.
2. Alchemers: Switch gunning proves to be the most OP DPS technique in the game if done right. Single-gun switching is a lot harder.
3. Cutter Swords: Requires the ability to plan your movements without diving into hazards. Could deal a lot of damage but you need to make sure all of your hits actually land
4. Needle gun: Powerful but you need to learn where to position yourself before you fire otherwise you're opening yourself to danger
5. Catalyzers: Similar to cutter swords; make sure you land ALL of your shots or else your damage is trash
6. Antigua: Better mobility than Needle Gun but easy to miss. Need to land all of your shots in order to maximize damage
7. Torto-fist Guns: Requires a bit of positioning but not too difficult to use. (I never used it myself)
8. Vortex Bombs: Small blast radius and requires a bit of positioning but nonetheless not too difficult to use.
Easiest Weapons:
1. Brandishes: Charge and hit. Out-damages all other weapons
2. Blaster: Just shoot. You can also utilize the switch gunning technique with Blasters but Alchemers have much higher damage output.
3. Pulsar: Bullet Streaming is the easiest technique in the game without having to reload
4. Blast Bombs: Place it and run. Good for speed runs
5. Haze Bombs: Just place it and wait for someone to walk into it. Low damage however.
6. Magnus: Charge and shoot, but requires more positioning than Brandishes.
7. Troika & Sealed Sword: Huge range and decent damage, but slow speed so requires a bit of timing.
8. Orbitgun

It's a very important but very subjective measure, for which I think that subjectivity belittles its importance.
Perhaps the more intriguing idea is that for each level of difficulty of use, there exists some amount of value to be gained wherein that weapon becomes a viable choice, and that moreover this value differs based on player skill level among other factors. There's also skill as it pertains to individual weapons to consider; if I'm skilled with acheron, tortofists have to offer something pretty good for me to forget about my training and jump on the tortofist ship.
i would disagree with putting orbitgun/mixer on easy to use, because it order for it to be viable you must weapon switch with it. Similar to alchemer, but still does more dps when switching by far
also magnus still requires skill with aiming and stuff

Something that seems lost here is that certain weapons in the wrong hands can make the game much harder for the people around them. The ones that first spring to mind are Pulsars because the excessive knockback can take mobs out of your reach, or worse, knock them into you. There are also minor flaws that can rustle peoples' jimmies like the screen-shaking from all of the Normal damage Blast Bombs and the Catalyzers' charge.
Other key details seem left out in exchange for classifying weapons only by their general play style. Callahan is much harder than W. Grave or Iron Slug because half of everything it targets is fast and hard to hit, and it is weak against 2/3 of turrets. Gran Faust is much harder than D. Avenger.
Lastly, where is Fang of Vog under hard weapons? Has useless combo attacks, but requires over-specializing your armor sets or getting defensive UVs to use the charge optimally, then turns out to be worse than most of the other elemental/fire weapons anyway, and still lights you on fire at inopportune times.
In my opinion bombs are very easy to use because you can only charge, as opposed to say brandishes where you constantly have to decide between charge or normal hits.
Also where would you rank the WARMASTER ROCKET HAMMER?

I would place mist bombs in the easy category. Not only are they very easy to use since you don't have to consider knockback at all, they can also make runs easy for everyone. A half-decent shivermist bomber can ensure that the party never needs to learn how to fight vanaduke, for instance.
Shivermist in particular is an extremely effective weapon that people mostly forego because it makes the game boring and they don't need it anymore.
It seems like a pretty good ranking.
However, there are multiple ways to measure easiness. Take Antigua-line guns for example. In some ways they are easy (many fast bullets in regular attacks) and in other ways hard (charge attack locks you in place for a long time, low damage means longer fights and more opportunities to get hurt, etc.). And alchemers are not very hard, if you're not trying to get huge DPS.
I'd say that your ranking is something like: "For whichever has higher potential --- regular attacks or charge attacks --- how hard is it to reach the weapon's potential?" Let me know if you disagree.